For marketing purposes/show of good faith to the public/whatever, certain "backward compatibility"/"side compatibility" was enshrined in the standards/specifications for DVD and later for Blu-ray (also Toshiba's dead HD DVD, if anyone cares any longer lol). This helped these formats get off the ground, as buyers could already have a library of discs that could be played in the new equipment, and it allowed consumers to have one box that could do multiple things. Win/win situation. It is in the standards/specifications, so unless they re-negotiate among all the member companies to remove such playback, it will continue to be required as long as any company wishes to be a licensee for the formats.
The DVD standard requires playback of
standard Redbook Compact Discs.
The Blu-ray standard requires playback of
standard Redbook Compact Discs, and DVDs that comply with the standard DVD-video discs.
The players are
not required to play home-burned discs, mp3 CDs, CD-RW, or other discs such as CD-i or SVCD or DVD-A or 3D Blu-ray, etc. However, many players will play a wide array of disc types beyond those required by the standards themselves.
If a DVD player or Blu-ray player won't play a
standard Redbook CD, then it is either
defective, or is
non-compliant to the licenses for DVD or Blu-ray, and thus isn't technically even a Blu-ray player or DVD player.
As
@Blumlein 88 mentioned earlier, some brands may not
list every compatible disc on the outside of the box. And an anecdote about someone supposedly having a player that won't play CDs doesn't mean much. The person could be attempting to play an improperly-burned disc, or an mp3 disc that plays in some devices but not in others, or one of a multitude of other possibilities. It could be a fake/unlicensed/non-compliant Blu-ray player, or defective, or the person put the disc in upside down. lol
Regarding rot, I have over 3000 CDs, and have never owned or personally seen a bad disc. So much for
my personal anecdote lol. Hyperion was known to have a problem with discs manufactured in a specific plant during a specific timeframe, but as far as I know, it's not a given that CDs will fail in that way. That said, it's not designed to be an archival format. "Forever" in "perfect sound forever" was a bit of marketing hyperbole. Certainly it's resilient compared to a cassette tape or LP that suffers wear/degradation with each play (I recall there was a turntable that used lasers rather than a needle, so I suppose that device would be an exception lol, but I've never seen one in action nor known of anyone who owned or used or saw one).
On the topic of this thread, I own three boxed opera sets on LP featuring Leontyne Price, which I found still factory-sealed and purchased to have as religious totems. I also bought the 20th Anniversary numbered LP of Mariah Carey's
Butterfly as a show of support for her after that New Year's Eve debacle a while back. Not that she noticed my gesture of good will. hahaha!